1. Field of the Invention
The present invention relates to an optical storage device including a sector mark detecting circuit for detecting a sector mark on an optical storage medium.
2. Description of the Related Art
In an optical storage device such as an optical disk device etc, an optical storage medium is irradiated with a light beam from an optical head, thus reading and writing information. An increase in storage capacity has been demanded of this type of optical storage devices.
FIGS. 14A and 14B are explanatory diagrams showing the prior art.
Each track on an optical disk is segmented into sectors. As shown in FIG. 14A, a head of each sector is provided with a sector mark for recognizing the start of the sector. The sector mark is composed of an easily distinguishable pattern of data. Provided subsequent to the sector mark are an identifier (ID) region and a data region including DATA FIELD. The identifier region has at least a VFO adjusting pattern and an address mark (AM). User data is written to the DATA FIELD. A header is written as an embossed pit by a disk medium manufacturer, and the user is unable to rewrite the header.
An ID signal is set in connection with an existence and non-existence of the embossed pit previously formed in the disk medium, and is detected from a variation in light quantity of the light beams on a detector. In the magneto-optic disk, a magnetizing direction of a recording film on the disk medium is conceived as a change in polarizing plane of the laser beams, and an MO signal is detected on the detector. Further, according to the phase change type optical disk, recording is effected by utilizing a phase change phenomenon of the recording film, and a DD signal is detected as a variation in light beam of the laser beam.
The identifier (ID) region is recognized by detecting the sector mark, and there are confirmed addresses of a track number, an ID number and a sector number. Then, accessing of the data region is executed. It is therefore required that the sector mark be detected with high accuracy.
As illustrated in FIG. 14B, the sector mark detecting circuit is constructed of a binarizing circuit 90 for binarizing the reading signal, and a recognizing circuit 91 for recognizing the sector mark from the output of the binarizing circuit 90. This binarizing circuit 90 imparts a gain to the reading signal and compares it with a predetermined slice level. A binarized signal is thereby obtained.
Binarization detecting conditions such as the gain and the slice level have hitherto been fixedly set.
There arise, however, the following problems inherent in the prior art.
First, the conditions for the optical detection becomes more strict as the storage capacity of the optical disk gets larger. For example, a scatter in terms of performance of a laser diode or a photo detector exerts an influence on the sector mark detection rate. Therefore, according to the prior art by which the same detecting condition is set in each device, the sector mark detection rate decreases, and a decline of an access time is caused.
Second, the optical disk device needs to deal with portable optical disks having different storage capacities. For instance, a 3.5 in. optical disk is classified into a 128 MB disk, a 230 MB disk, a 540/640 MB disk and a 1.3 GB disk. In the conventional sector mark detecting circuit, the same detecting condition is set in the portable optical disks having the different storage capacities, and consequently the sector mark detection rate decreases as well as causing the decline of the access time. Namely, with an enhancement of the storage density, a data density/track density rises, and a noise quantity increases. The sector mark detection rate is thereby decreased. For example, the 128 MB optical disk has 25 sectors per track. The 540 MB optical disk, however, has 84 sectors per track, which is more than 3-fold strictness in terms of timing.